AC servomotors are becoming used for middle-sized injection molding machines heretofore driven by hydraulic actuators (clamping force >3.5 MN) that have high precision, quick response and higher power which are obtained by performance improvements of permanent magnets and cost reductions.
An injection molding machine consists of a plasticizier in which resin pellets are melted by friction heat generated by plasticizing screw revolution and stored at the end of a barrel, an injector in which an amount of melted polymer is injected into a metal mold at a given velocity and a given dwell pressure is applied, and a clamper in which the metal mold is clamped and opened, all using AC servomotors drive system. FIG. 2 shows a view of an injection molding mechanism using an AC servomotor.
On an injection machine base which is fixed on the ground, a movable base is located which moves on a linear slider and both the bases are not shown in FIG. 2. All parts except a metal mold 1 shown in FIG. 2 are mounted on the movable base. By sliding the movable base, the top of a barrel 2 is clamped on the metal mold 1 and vice versa the top of the barrel 2 is separated from the metal mold 1. FIG. 2 shows a mode in which the top of the barrel 2 is clamped on the metal mold 1 before melted polymer being injected into the metal mold 1.
On the movable base, a barrel 2, a servomotor 3, a reduction gear 4, a ball screw 5 and a bearing 6 are fixed. A nut 7 of the ball screw 5, a moving part 8, a screw 9 and a pressure detector 10 such as a load cell consist of an integral structure. The moving part 8 is mounted on a linear slider 11 so that the integral structure is moved back and forth by the movement of the nut 7 of the ball screw 5.
Rotation of the servomotor 3 is transferred to the ball screw 5 which magnifies linear force through the reduction gear 4 and rotation of the ball screw 5 is converted to a linear motion of the nut 7 of the ball screw 5 and a linear motion of the screw 9 and pressure application to melted polymer are realized through the moving part 8. Position of the screw 9 is detected by a rotary encoder 12 mounted on the servomotor 3. Pressure applied to the melted polymer at the end of the barrel 2 is detected by the pressure detector 10 mounted between the nut 7 and the moving part 8. A cavity 13 in the metal mold 1 is filled up with melted polymer by a movement of the screw 9.
Mold good manufacturing consists of injection and dwell pressure application. In the injection process, polymer melt must be injected into the cavity 13 as fast as possible so that temperatures of polymer in the cavity become homogeneous. However, as excessive injection velocity brings about excessive polymer pressure and mold defects, polymer pressure in the injection process is constrained under a given pressure limit pattern. In the pressure application process following the injection process, a given pressure pattern is applied for each given duration at the polymer in the cavity during cooling in order to supply a deficiency due to polymer shrinkage. Therefore, the following two requirements are given to the injection velocity pattern and the pressure application pattern.    (1) In the injection process, a given injection velocity pattern is realized and at the same time injection pressure is constrained under a given pressure limit pattern in terms of mold good quality.    (2) In the pressure application process, a given pressure pattern is realized and at the same time injection velocity is constrained under a given velocity limit pattern in terms of safety operation.
In the injection process (time 0˜t1) shown in FIG. 3(a), injection velocity control is carried out by giving injection velocity command shown in FIG. 3(b) to realize a given injection velocity pattern. However, injection pressure has to be controlled lower than a given pressure limit pattern shown in FIG. 3(c). Vertical scales 100% shown in FIGS. 3(b) and (c) indicate maximum values of injection velocity and injection pressure, respectively.
In the pressure application process (time t1˜t2) shown in FIG. 3(a), pressure application control is carried out by giving pressure application command shown in FIG. 3(c) to realize a given pressure application pattern. However, injection velocity has to be controlled lower than a given injection velocity limit pattern shown in FIG. 3(b).
FIG. 4 shows a block diagram of a controller which realizes the above two requirements (1) and (2) (paragraph {0008}) (patent literature PTL 1). The controller consists of an injection controller 20 and a motor controller (servoamplifier) 40.
The injection controller 20 executes a control algorithm at a constant time interval Δt and a discrete-time control is used. The injection controller 20 consists of an injection velocity setting device 21, a transducer 22, a pulse generator 23, an analog/digital (A/D) converter 25, an injection pressure setting device 26, a subtractor 27, a pressure controller 28, and a digital/analog (D/A) converter 29. The pressure detector 10 is connected to the injection controller 20.
The injection velocity setting device 21 feeds a time sequence of injection velocity command Vi* to the transducer 22. The transducer 22 calculates screw displacement command Δxv* for the screw 9 which has to move during the time interval Δt by the following equation (1).{Math. 1}Δxv=Vi*Δt  (1)
The command Δxv* is fed to the pulse generator 23.
The pulse generator 23 feeds a pulse train 24 corresponding to the command Δxv*. The pulse train 24 is fed to a pulse counter 41 in the motor controller 40.
The pressure detector 10 feeds an injection pressure signal Pi to the injection controller 20 through the A/D converter 25. The A/D converter 25 feeds the pressure signal Pi to the subtractor 27.
The injection pressure setting device 26 feeds a time sequence of injection pressure command Pi* to the subtractor 27. The subtractor 27 calculates a pressure control deviation ΔPi by the following equation (2).{Math. 2}ΔPi=Pi*−Pi  (2)
The control deviation ΔPi is fed to the pressure controller 28.
The pressure controller 28 calculates a motor current demand ip* from ΔPi by using PID (Proportional+Integral+Derivative) control algorithm and feeds the demand ip* to the motor controller 40 through the D/A converter 29.
Next, the motor controller 40 is explained. The motor controller 40 consists of pulse counters 41 and 44, an A/D converter 42, a comparator 43, subtractors 45 and 48, a position controller 46, a differentiator 47, a velocity controller 49 and a PWM (Pulse Width Modulation) device 50. The motor controller 40 is connected to the servomotor 3 equipped with the rotary encoder 12.
In the motor controller 40 the demand from the injection controller 20 is fed to the comparator 43 through the A/D converter 42.
The pulse counter 41 accumulates the pulse train 24 from the injection controller 20 and obtains screw position demand x* and feeds the demand x* to the subtractor 45. The pulse counter 44 accumulates the pulse train from the rotary encoder 12 and obtains actual screw position x and feeds the position signal x to the subtractor 45.
The subtractor 45 calculates a position control deviation (x*−x) and feeds the position deviation to the position controller 46. The position controller 46 calculates velocity demand v* by the following equation (3) and feeds the demand v* to the subtractor 48.{Math. 3}v*=Kp(x*−x)  (3)
where Kp is a proportional gain of the position controller 46.
The rotary encoder 12 feeds a pulse train to the differentiator 47 and to the pulse counter 44. The differentiator 47 detects actual screw velocity v and feeds the velocity signal v to the subtractor 48.
The subtractor 48 calculates a velocity control deviation (v*−v) and feeds the velocity deviation to the velocity controller 49. The velocity controller 49 calculates a motor current demand iv* by the following equation (4) and feeds the demand iv* to the comparator 43.
                    {                  Math          .                                          ⁢          4                }                                                                      i          v          *                =                                            K                              P                ⁢                                                                  ⁢                υ                                      ⁡                          (                                                v                  *                                -                v                            )                                +                                                    K                                                      P                    ⁢                                                                                  ⁢                    v                                    ⁢                                                                                                                    T                                  I                  ⁢                                                                          ⁢                  v                                                      ⁢                          ∫                                                (                                                            v                      *                                        -                    v                                    )                                ⁢                                  ⅆ                  t                                                                                        (        4        )            
where KPv and TIv are a proportional gain and an integral time constant of the velocity controller 49, respectively. In the motor controller 40 a position control loop has a minor loop of velocity control.
The comparator 43 to which both motor current demands iv* and ip* from the velocity controller 49 and the pressure controller 28, respectively, are fed, selects a lower current demand i* of iv* and Ip* and feeds the lower demand i* to the PWM device 50. The PWM device 50 applies three-phase voltage to the servomotor 3 so that the servomotor 3 is driven by the motor current i*. The comparator 43 restricts motor current demand iv* decided by injection velocity control loop to motor current demand ip* decided by pressure control loop.
Next, it can be shown by using FIG. 3 that the above described two requirements (1) and (2) (paragraph {0008}) are realized by the comparator 43.
In FIG. 3, transfer time t1 from injection to pressure application is specified by an operator, so the time t1 should coincide with the time at which the cavity is filled up with polymer melt, but it is difficult for an operator to set the time t1 at the exact time. Firstly the finishing time t1 of injection process is supposed to be set by an operator before the time at which the cavity 13 is filled up with polymer melt actually. When the time reaches t1 and pressure application process starts, actual pressure Pi is lower than a set value Pi* because the cavity is not yet filled and motor current demand ip* fed by the pressure controller 28 increases so that pressure Pi is increased to the set value Pi*.
If demand ip* is selected as a final motor current demand, injection velocity increases rapidly because the cavity 13 is still filling. Actual velocity could exceed the velocity limit shown in FIG. 3(b). However, even if demand ip* exceeds iv* when pressure application starts, the comparator 43 always selects a lower demand of ip* and iv* and selects the lower demand iv* as a final motor current demand and limits velocity. That is, by the comparator 43 pressure application control is transferred to velocity limit control and the above described requirement (2) (paragraph {0008}) is always satisfied.
Secondly the time t1 is supposed to be set by an operator after the time at which the cavity 13 is filled up actually. Even when the cavity is filled up, injection process continues and injection velocity control is carried out. But actual screw speed slows as the cavity is already filled up and so motor current demand iv* fed by the velocity controller 49 is increased to maintain the injection velocity.
If demand iv* is selected as a final motor current demand, injection pressure increases rapidly because filling is completed and so actual pressure may exceed the pressure limit shown in FIG. 3(c). However, even if demand iv* exceeds ip* in injection process when filling is completed, the comparator 43 always selects a lower demand of iv* and ip* and selects the lower demand ip* as a final motor current demand and limits pressure in injection.
That is, by the comparator 43 injection velocity control is transferred to pressure limit control and the above described requirement (1) (paragraph {0008}) is always satisfied.
The object of the comparator 43 is to constrain an excessive pressure variation or an excessive velocity variation generated by a mutual transfer between the injection velocity control and the injection pressure control and a minimum selector (a low selector) which selects a smaller signal of inputted two signals found in patent literatures PTL 2 and PTL 3 has the same object as the above comparator 43.
In the controller shown in FIG. 4, the pressure detector 10 is absolutely necessary. Patent literatures PTL 4˜PTL 12 are applications of the apparatus and method for pressure control of injection molding machines without using the pressure detectors.
In patent literature PTL 4 for hydraulic actuator driven injection machines, polymer characteristics formula which gives the relational expression among polymer pressure, polymer temperature and polymer specific volume is used and the required polymer pressure is calculated by inputting measured polymer temperature and polymer specific volume which is decided from the desired value of mold good weight. Then by using initial temperatures of metal mold and polymer at the start of pressure application process and the above required polymer pressure, the required set value of pressure application is derived through an approximate expression. The pressure application set value is fed to the hydraulic servovalve amplifier as the voltage command converted and the set value of applied pressure is realized by the hydraulic pressure of hydraulic cylinder piston.
In patent literature PTL 5, in order to detect polymer pressure in the cavity the pressure is applied to a plunger which moves back and forth in the cavity and is connected with a ball screw mechanism whose nut is rotated by a servomotor. In injection and pressure application process the servomotor holds the position of the plunger to which the polymer pressure is applied and the servomotor current is detected by a current transducer and the detected current is converted to the polymer pressure in the cavity. The position of the plunger is detected by a rotary encoder equipped with the servomotor.
In patent literature PTL 6, in order to detect polymer pressure in the cavity a disturbance observer is used for a servomotor drive system which moves a plunger back and forth in the cavity. In injection and pressure application process the pressure is applied to the plunger and the servomotor drive system holds the position of the plunger. Then the disturbance observer estimates a load torque of the servomotor by using a motor speed signal and a motor torque command signal. The pressure in the cavity is obtained from the estimated load torque. The arithmetic expressions of the observer are shown in the literature. The method by which pressure in the cavity is obtained directly by using detected servomotor current or motor torque command, is also shown in the literature.
In patent literature PTL 7, firstly a function which estimates a polymer pressure in the cavity by using injection screw drive force and injection velocity, is decided. In the actual control actions, feature size of mold good, polymer data, real-time data of screw drive force and injection velocity are fed to the above function and the real-time estimated pressure in the cavity is obtained. Injection velocity is controlled by deviation of the estimated pressure from the reference value. The procedures of obtaining the above exact function are shown in the literature.
In patent literature PTL 8, the pressure control apparatus is realized in which an observer for a servomotor drive system estimates a polymer pressure and the estimated pressure is used as a detected signal for the pressure control. The observer is fed by a motor speed in an injection process and the total friction resistance in an injection mechanism and outputs the estimates of motor speed and polymer pressure. The observer is applied to the following two models.    (1) A servomotor drives an injection screw through a linear motion converter such as a ball screw only.    (2) A servomotor drives an injection screw through a belt pulley reduction gear and a linear motion converter.
In the observer model (1), the friction resistance consists of a dynamic friction resistance and a static friction resistance over an injection mechanism. In the observer model (2), the friction resistance consists of a dynamic friction resistance only which is defined as a sum of a velocity dependent component and a load dependent component.
In the observer model (1) the polymer pressure is assumed to be constant. In the observer model (2) the observer outputs not only the estimates of motor speed and polymer pressure but also the estimates of pulley speed at load side, belt tension and force applied to polymer melt by a screw. It is assumed that the belt is elastic and the time rate of change in polymer pressure is proportional to pulley speed at load side, to pulley acceleration and to force applied to polymer by a screw. The force applied to polymer melt by a screw is assumed to be constant.
In patent literature PTL 9, an injection velocity and pressure control apparatus is realized in which an observer for a servomotor drive system estimates the load torque generated by polymer pressure. The observer is fed by motor speed signal and motor current command signal and outputs the estimates of motor speed, load torque and position difference between a motor shaft and a load side shaft. The model of the observer consists of the servomotor drive system which moves a screw through a belt pulley reduction gear. The pressure value converted from the estimated load torque obtained by the observer is used as a detected pressure signal.
In patent literature PTL 10, the observer for the model (2) in PTL 6 (paragraph {0041}) is used and the pressure control method is invented. The method uses a motor speed and the estimates of pulley speed at load side, belt tension and polymer pressure as feedback signals of state variables. These estimates are obtained by the observer. Another control method is invented, in which the servomotor torque command is decided by the above four state variables.
In patent literature PTL 11, a method is invented, in which the estimate of load torque applied by polymer pressure is obtained by using an inverse model of a transfer function whose inputs are a motor generated torque and a load torque and output is a motor speed. The inverse model is fed by a motor speed and a motor generated torque and derives the estimate of load torque. The polymer pressure is obtained from the estimate of the load torque. The inverse model requires the high-order differentiation.
In patent literature PTL 12, a method is invented, in which a polymer pressure is estimated by using a motion equation of an integral structure consisted of a moving member and a screw and by using measured values of a motor rotation speed and a motor torque. The motion equation of the integral structure converted to the motor axis is shown by the following equation (5). The motor shaft and the ball screw shaft are coupled through a belt pulley reduction gear.
                    {                  Math          .                                          ⁢          5                }                                                                                  J            TOT                    ⁢                                    ⅆ              ω                                      ⅆ              t                                      =                              T            2                    -                                    l                              2                ⁢                                                                  ⁢                π                                      ⁢                          1                                                e                  S                                ⁢                                  e                  B                                                      ⁢                                          N                MP                                            N                SP                                      ⁢                          (                                                                    A                    BARREL                                    ⁢                                      P                    MELT                                                  +                                  F                  LOSS                                            )                                -                      T            U                                              (        5        )            
where t: Time variable, JTOT: Reduced total moment of inertia at motor axis, ω: Angular velocity of motor, T2: Motor torque, l: Ball screw lead, eS: Ball screw efficiency, eB: Belt pulley efficiency, NMP: Pulley diameter at motor side, NSP: Pulley diameter at ball screw side, ABARREL: Barrel section area, PMELT: Polymer pressure, FLOSS: Total friction force acting on the integral structure due to a friction at a ball screw mechanism and due to a friction between a barrel surface and a screw, TU: Torque loss due to a friction force at a support rail of the integral structure. Angular velocity of motor ω and motor torque T2 are measured. Angular acceleration of motor α−dω/dt is obtained by a numerical time differential operation for an angular velocity of motor ω. If FLOSS, TU etc. are known, PMELT is obtained by using the following equation (6).
                    {                  Math          .                                          ⁢          6                }                                                                      P          MELT                =                              1                          A              BARREL                                ⁢                      {                                                                                2                    ⁢                                                                                  ⁢                    π                                    l                                ⁢                                                      N                    SP                                                        N                    MP                                                  ⁢                                  e                  S                                ⁢                                                      e                    B                                    ⁡                                      (                                                                  T                        2                                            -                                                                        J                          TOT                                                ⁢                        α                                            -                                              T                        U                                                              )                                                              -                              F                LOSS                                      }                                              (        6        )            
In the estimation method of polymer pressure by equation (6), an error of angular acceleration of motor a due to the numerical differential operation and errors of FLOSS and TU bring about the error of the estimated polymer pressure.
The common object of inventions described in patent literatures PTL 4˜PTL 12 which detect polymer pressure without using a pressure detector is to avoid the following disadvantages.    (1) A highly reliable pressure detector is very expensive under high pressure circumstances.    (2) Mounting a pressure detector in the cavity or the barrel nozzle part necessitates the troublesome works and the working cost becomes considerable.    (3) Mounting a load cell in an injection shafting alignment from a servomotor to a screw complicates the mechanical structure and degrades the mechanical stiffness of the structure.    (4) A load cell which uses strain gauges as a detection device necessitates an electric protection against noise for weak analog signals. Moreover the works for zero-point and span adjustings of a signal amplifier are necessary (patent literature PTL 13).
Patent literatures PTL 14 and PTL 15 are inventions concerning the pressure control at electric-motor driven injection molding machines and both necessitate pressure detectors. In patent literature PTL 14, the concept of virtual screw velocity ω1 is introduced in equation (1) of the description of PTL 14 and equation (1) is based on the point of view which the pressure control is conducted by screw position control. An exact control method is realized by using virtual velocity ω1 as a parameter which compensates the pressure loss due to the nonlinear loss which results in the difference between the pressure corresponding to motor generated torque and pressure set value. The disturbance observer outputs the estimate of virtual velocity ω1 so that the difference between the pressure detected by a load cell and the estimated pressure becomes zero by using the error between detected pressure and the estimated pressure. Patent literature PTL 15 is a prior application of PTL 14 and it is different from PTL 14 in the observer structure.